Last week Samsung issued the largest mobile device recall to date, when it warned users of its Galaxy Note 7 that a battery defect was causing the devices to catch fire and explode. Samsung attributed the problem to a flaw in some of the lithium-ion batteries made by one of its manufacturers — forcing the company to recall 2.5 million of the devices. This isn’t an isolated problem, in recent years HP, Lenovo, Sony, Dell, Apple and many other electronics companies have issued recalls related to dangerous battery malfunctions.
Smaller devices with greater processing power are pushing batteries to the limit. Earlier this week a group of Drexel researchers published in Nature Communications what amounted to a roadmap for better energy storage device development. But until that happens, it’s important for consumers to understand why batteries malfunction and what warning signs they might be giving you before it happens.
Vibha Kalra, PhD, an associate professor of chemical and biological engineering in the College of Engineering, who studies and develops energy storage materials took some time to break down battery behavior for the News Blog.
What sort of malfunctions could cause lithium-ion batteries like this to explode/catch fire?
There are several mechanisms by which a battery could fail. Batteries need to be kept within a certain temperature and voltage range for safe operation. Any battery abuse or usage beyond these windows can be dangerous. For example, high external temperatures are not desirable for the flammable electrolytes in the batteries. In addition, high temperatures can melt the separator. The separator physically separates the positive and negative sides and prevents current flow between the electrodes. So if it melts it can cause short-circuiting. Further heat can cause the cathode side to release oxygen, which can trigger burning of the electrolyte. In addition manufacturing defects such as metal particle contaminants or a defect with the separator layer (that physically separates positive and negative sides of the battery) can result in a short circuit leading to overheating of the battery.
Is this just part of the natural risk associated with batteries becoming smaller and smaller, or does there have to be a pretty significant failure or mistake for something like this to occur?
A defect-free battery that is used within the recommended voltage and temperature windows as well as disposed appropriately is safe to use. Nevertheless, the components used in li-ion batteries are inherently unsafe, so even a “minor” manufacturing defect or battery abuse can be dangerous. The electrolyte is flammable, the cathode oxides can produce oxygen on overheating, which can trigger burning. Metal particle contaminants in the manufacturing process can lead to short circuits. So from research and development point of view, new materials can help in reducing this risk, for example solid electrolytes (instead of flammable liquid electrolytes) can reduce part of the risk, but it comes with a tradeoff of slower ion transport and lower power. So development of new solid electrolytes along with development of novel electrode architectures that can allow efficient 3D infusion of the solid electrolyte at the nanoscale to allow efficient ion transport will help minimize this tradeoff.
The flaw in this case involved the negative and positive electrodes combing together — what happens in a battery when this occurs?
If negative and positive sides of a battery combine either due to a material failure or due to defect in the cell assembly- it leads to short circuiting of the battery (i.e. current flow between the electrodes) resulting in overheating, and possible fire/explosion.
How have batteries been designed to prevent this disastrous anomaly from happening?
Batteries have a thin porous material layer called the separator in between the positive and negative electrodes whose role is to physically separate the two sides and prevent any direct current flow between them while still allowing the transport of lithium ions. In addition, battery manufacturers incorporate several safety features in the batteries to prevent inappropriately high current and high charge voltage. One example is a fuse that shuts off the current flow if the battery temperature rises above a certain level. Another example of a safety feature is that individual cells within a battery pack have dividers between them to prevent the spread of one cell failure to adjacent cells so as to contain the damage. However, if there are manufacturing defects internal to a cell – those are often harder to control.
How does rapid-charge technology, as it continues to be developed and used in devices, bring with it new design/safety challenges?
Faster charging means more heating, which makes the need to develop newer more robust materials even more critical.
Kalra is a professor in the Department of Chemical and Biological Engineering, you can learn more about her research here: www.chemeng.drexel.edu/kalraresearchgroup/
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